1. Field of the Invention
The present invention relates in general to a marine drive, and in particular to an exhaust system of a marine drive which employs a through-the-hub exhaust discharge arrangement.
2. Description of Related Art
An outboard motor commonly discharges at least a portion of its engine exhaust into the body of water in which the outboard motor is operated in order to silence exhaust noise. For this purpose, an exhaust system of the outboard motor delivers engine exhaust below the water level through an exhaust passage formed in a lower unit and discharges the exhaust gases through the hub of the propeller.
Some outboard motors also discharge a portion of the exhaust gases upstream of the propeller when operating at idle or at low speeds. The exhaust gases discharged in the vicinity of the propeller aerate the water so as to reduce water resistance on the propeller. The propeller consequently accelerate more quickly.
Although such exhaust systems improve acceleration from low speeds, the rapid braking effect created by quickly reversing propeller rotation may be sacrificed to some extent with these systems. When braking by quickly reversing propeller rotation, the forward momentum of the watercraft often carries the propeller blades into the discharge exhaust gas stream (which discharges between the lower unit and the propeller blades), even though the propeller blades are rotating in reverse. The blades consequently cavitate, which decreases the braking efficiency of the propeller. The outboard motor thus may not provide the rapid braking force preferred by some boat operators.